This invention relates to lasers and in particular to narrow-band ArF excimer lasers.
Krypton-Fluoride (KrF) excimer lasers are currently becoming the workhorse light source for the integrated circuit lithography industry. The KrF laser produces a laser beam having a narrow-band wavelength of about 248 nm and can be used to produce integrated circuits with dimensions as small as about 180 nm. The Argon Fluoride (ArF) excimer laser is very similar to the KrF laser. The primary difference is the laser gas mixture and a shorter wavelength of the output beam. Basically, Argon replaces Krypton and the resulting wavelength of the output beam is 193 nm. This permits the integrated circuit dimensions to be further reduced to about 120 nm. A typical prior-art KrF excimer laser used in the production of integrated circuits is depicted in FIG. 1 and FIG. 2. A cross section of the laser chamber of this prior art laser is shown in FIG. 3. A pulse power system 2 powered by high voltage power supply 3 provides electrical pulses to electrodes 6 located in a discharge chamber 8. Typical state-of-the art lithography lasers are operated at a pulse rate of about 1000 to 2000 Hz with pulse energies of about 10 mJ per pulse. The laser gas (for a KrF laser, about 0.1% fluorine, 1.3% krypton and the rest neon which functions as a buffer gas) at about 3 atmospheres is circulated through the space between the electrodes at velocities of about 1,000 inches per second or greater. This is done with tangential blower 10 located in the laser discharge chamber. The laser gases are cooled with a heat exchanger 11 also located in the chamber and a cold plate (not shown) mounted on the outside of the chamber. The natural bandwidth of the excimer lasers is narrowed by line narrowing module 18. Commercial excimer laser systems are typically comprised of several modules that may be replaced quickly without disturbing the rest of the system. Principal modules include:
Laser Chamber Module,
Pulse Power System with: high voltage power supply module, commutator module and high voltage compression head module,
Output Coupler Module,
Line Narrowing Module,
Wavemeter Module,
Computer Control Module,
Gas Control Module,
Cooling Water Module
Electrodes 6 consist of cathode 6A and anode 6B. Anode 6B is supported in this prior art embodiment by anode support bar 44 which is shown in cross section in FIG. 3. Flow is clockwise in this view. One corner and one edge of anode support bar 44 serves as a guide vane to force air from blower 10 to flow between electrodes 6A and 6B. Other guide vanes in this prior art laser are shown at 46, 48 and 50. Perforated current return plate 52 helps ground anode 6B to the metal structure of chamber 8. The plate is perforated with large holes (not shown in FIG. 3) located in the laser gas flow path so that the current return plate does not substantially affect the gas flow.
A peaking capacitor comprised of an array of individual capacitors 19 is charged prior to each pulse by pulse power system 2. During the voltage buildup on the peaking capacitor, two preionizers 56 weakly ionize the lasing gas between electrodes 6A and 6B and as the charge on capacitors reach about 16,000 volts, a discharge across the electrode is generated producing the excimer laser pulse. Following each pulse, the gas flow between the electrodes of about 1 inch per millisecond, created by blower 10, is sufficient to provide fresh laser gas between the electrodes in time for the next pulse occurring one millisecond later.
In a typical lithography excimer laser, a feedback control system measures the output laser energy of each pulse, determines the degree of deviation from a desired pulse energy, and then sends a signal to a controller to adjust the power supply voltage so that energy of subsequent pulses are close to the desired energy. In prior art systems, this feedback signal is an analog signal and it is subject to noise produced by the laser environment. This noise can result in erroneous power supply voltages being provided and can in turn result in increased variation in the output laser pulse energy.
These excimer lasers are typically required to operate continuously 24 hours per day, 7 days per week for several months, with only short outages for scheduled maintenance. One problem experienced with these prior-art lasers has been excessive wear and occasional failure of blower bearings.
A prior art wavemeter utilizes a grating for coarse measurement of wavelength and an etalon for fine wavelength measurement and contains an iron vapor absorption cell to provide an absolute calibration for the wavemeter. This prior art device focuses the coarse signal from the grating onto a linear photo diode array in the center of a set of fringes produced by the etalon. The center fringes produced by the etalon are blocked to permit the photo diode array to detect the coarse grating signal. The prior-art wavemeter cannot meet desired speed and accuracy requirements for wavelength measurements.
A need exists in the integrated circuit industry for a modular, reliable, production line quality ArF excimer laser in order to permit integrated circuit resolution not available with KrF lasers.
The present invention provides a reliable modular production quality ArF excimer laser capable of producing laser pulses at repetition rates in the range of 3,000 to 4,000 Hz or greater with pulse energies in the range of about 2 mJ to 5 mJ or greater with a full width half, maximum bandwidth of about 0.4 pm or less and dose stability of less than 0.4 percent. Using this laser as an illumination source, stepper or scanner equipment can produce integrated circuit resolution of 0.10 xcexcm (100 nm) or less. Replaceable modules include a laser chamber; a modular pulse power system; and a line narrowing module. For a given laser power output, the higher repetition rate provides two important advantages. The lower per pulse energy means less optical damage and the larger number of pulses for a specified illumination dose means better dose stability.
Important improvements have been provided in the pulse power unit to produce faster charging. These improvements include an increased capacity high voltage power supply, an improved commutation module that generates a high voltage pulse from the capacitors charged by the high voltage power supply and amplifies the pulse voltage 23 times with a very fast voltage transformer having a secondary winding consisting of a single four-segment stainless steel rod. A novel design for the compression head saturable inductor (referred to herein as a xe2x80x9cpots and pansxe2x80x9d design) greatly reduces the quantity of transformer oil required and virtually eliminates the possibility of oil leakage which in the past has posed a hazard.
Improvements in the laser chamber permitting the higher pulse rates and improved bandwidth performance include the use of a single preionizer tube.
Improvements in the resonance cavity of preferred embodiments of the present invention include a line narrowing module with CaF prism beam expanders and a grating specially coated for UV damage resistance. Preferred embodiments comprised output couplers having substantially increased reflectivity over prior art designs.
A newly designed wavemeter including a computer processor programmed with an algorithm for controlling wavelength measurements and computing wavelengths at rates sufficient for feedback control of the wavelength of the output laser beam at rates of 3,000 Hz or faster. In a preferred embodiment a vapor cell with platinum vapor providing a reference absorption line for system calibration.
Other embodiments of the present invention include ceramic bearings. Optionally magnetic bearings may be utilized. Reaction forces on the bearings may be reduced by providing an aerodynamic contour on the anode support bar. Other improvements include use of acoustic baffles for laser chambers producing disruptive acoustic shock waves.